Due to variations in individual fuel injectors and the need for consistent performance among all injectors used in an engine, various techniques have been employed in attempts to match performance characteristics of injectors for use as a set in an engine. For example, manufacturing processes must observe strict tolerances to help achieve uniformity of injector specifications. In spite of this, performance differences still exist, which may be addressed by electronic trimming (e-trim) of the injectors on a test bench, and subsequent matching of injector sets for use in an engine.
The above techniques, however, can only be effective to a certain extent. Since manufacturing strategies and test bench e-trim processes are conducted under restricted conditions, the reality of use in an engine under a wide variety of operating conditions reveals shortcomings of injector-to-injector performance that are not evident otherwise. In particular, injector performance during low idle conditions, i.e., when very small fuel quantities are being demanded of the injectors, may not be as optimal as performance under operating conditions which more closely resemble test bench conditions.
In addition, conditions related to the operation of the engine itself, such as changing temperature conditions (both with respect to time and location on the engine) and low pressure dynamics (particularly during low idle) contribute to variations in injector performance characteristics.
Attempts have been made in the past to trim injectors after installation on an engine. Typically, these methods involve cutting out operation of one injector at a time and monitoring changes in fuel delivery to the remaining injectors. The changes in fuel delivery are analyzed to determine trim requirements for each injector, typically in the form of varying injection duration. It has been found, however, that the changes in fuel delivery may not offer enough resolution for analysis, particularly during low idle conditions when the amount of fuel demanded is very small. The problem is compounded, moreover, when engine operations involves delivery of multiple shots of fuel, such as a split injection mode, thus making individual fuel deliveries much smaller.
An alternative to measuring changes in fuel delivery during cutout testing is to monitor changes in engine speed, a parameter that may offer more resolution for incremental changes. For example, in U.S. Pat. No. 5,113,830, Haines discloses a control system which, under certain operating conditions, monitors changes in engine speed as individual injectors are disabled. Haines, however, specifically limits this form of testing to operating conditions in which no speed governing takes place. Therefore, testing cannot be conducted, for example, at low idle speeds, during which the system of Haines is in a “two speed governing mode”. Furthermore, Haines conducts testing by disabling injectors completely and does not perform tests in which only injector events, such as particular shots of multi-shot injections, are halted.
The present invention is directed to overcoming one or more of the problems as set forth above.